1. Field of the Invention
The present invention concerns a method of treating mother liquor of reaction in terephthalic acid production through a liquid phase oxidation of paraxylene.
2. State of the Art
Industrial production of terephthalic acid generally employs a process of oxidizing paraxylene, the raw material, in the presence of a salt or salts of heavy metal(s) such as Co or Co+Mn as a catalyst, with molecular oxygen, in liquid medium of lower aliphatic acid, usually acetic acid. The formed terephthalic acid precipitates in reaction solution and is recovered from the solution by means of separation such as filtration or centrifuge. The mother liquor thus given contains, in addition to the acetic acid medium, catalyst, terephthalic acid which is dissolved or suspended therein, and reaction by-products such as water, paratolyl acid, 4-carboxy benzaldehyde, isophthalic acid, benzoic acid and high polymers.
Heretofore, the mother liquor has been treated by, firstly evaporation procedure to recover major portion of the solvent, acetic acid, and then, by evaporating the concentrated slurry in a thin-film evaporator to recover the rest of the solvent. The thick slurry obtained solidifies after cooling, and heavy metal components in the solid are extracted to regenerate and reuse as the catalyst.
In general, operation of a thin-film evaporator is delicate and troublesome; particularly in treatment of the above described terephthalic acid production process, it is difficult to continue smooth operation, probally due to some contents of the matter to be treated. Consequently, operation of the process, including the oxidation step, has been often interrupted. Moreover, the thick slurry from the thin-film evaporator solidifies through cooling to form a solid mass, which necessitates crushing and atomizing of the mass for the extraction of the heavy metal components.
As solutions for these problems, there has been proposed, to date, addition of an aliphatic ketone to the mother liquor (Japanese Patent Disclosure No. 136634/1976), concentration of the mother liquor to some extent and then addition of aromatic hydrocarbon under heating to distill out acetic acid and to obtain solid components (Japanese Patent Disclosure No. 136635/1976). According to these improved methods, it is possible to realize more efficient solid-liquid separation and to get a solid which can be treated easily. However, introduction of the substances of no direct necessity for the reaction into the process brings about new problem of necessary separation step for the substances.